Plant Speciation – Part 2

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AA BB CC EEDD FF

Spring 2010

Major topics

• Variation in plant populations and species (1)

• Gene flow and reproductive isolating barriers (1)

• Speciation mechanisms (modes) (2)

• Species concepts (2)

Speciation

• Speciation = permanent severing of two or more sets of populations so that migrants from one population system would be at a disadvantage when entering the other

• Disadvantages: lack of mates (reproductive isolation); less competitive (adaptation)

• Speciation may result from adaptive changes or chance events (or both)

Modes of Speciation

• Traditionally viewed as the gradual accumulation of differences via selection to produce two isolated sets of populations that cannot interbreed

• Allopatric speciation

Disruptive selection is the beginningof gradualistic

speciation.

• Selection is against the mean values of the phenotypic range.

● Geographic isolation is expected.

• Reproductive barriers complete the process in most organisms.

Allopatric Speciation

Population A

Population B

geneflow

A

B

geographicbarrier arises

Species A

Species B

reproductiveisolation

timeD.S.

D.S.

Allopatric SpeciationSpecies A

Species B

geneflow

allopatric speciation =geographic isolation

followed byreproductive isolation

local speciation =peripheral populations

become neospecies; thesemay become fully reproductively

Isolated species

Sympatric Speciation

• The other main mode of speciation

• But first we need to review chromosomes and chromosomal changes

Basic Chromosome Morphoplogy

Chromosome NumberHaploid number – lowest chromosome number in the spores or

gametes (egg or sperm) (written as n)

Diploid number – lowest chromosome number in the somatic (non-sex) cells (written as 2n); for a given species, should be 2x the gametes

In angiosperms, chromosome numbers range from 2n = 4 to 2n = 250; average is about 2n = 26. Ferns can have much higher numbers.

For most species, chromosome numbers are fixed, and all individuals have the same number.

In some cases, chromosome numbers show instability, and vary

widely within certain groups. Can be chromosome number variation within a single species or even a single individual (not counting gametes).

Chromosome Sets - Ploidy

• POLYPLOIDY – Presence of the three or more sets of chromosomes in somatic cells (often written as X—2X, 3X, 4X, etc.).

• ANEUPLOIDY – Loss or gain of whole chromosomes.

Aneuploidy in Claytonia virginica

(Portulacaceae)

2n = 12, 14, 16, 17-37, 40, 42, 44, 46, 48, 50, 72, 81, 85, 86, 87, 91, 93, 94, 96, 98, 102, 103, 104, 105, 110, 121, 173, 177, 191

Spring Beauty

Polyploidy:An example from Rumex

(Polygonaceae)

2n n X Ploidy level

R. sanguineus 20 10 2X diploid

R. obtusifolius 40 20 4X tetraploid

R. hydrolapathum 120 60 12X dodecaploid

Meiosis vs. Mitosis

Modes of Speciation

• Speciation may occur without geographic isolation, but reproductive isolation is still necessary

• Sympatric speciation (through polyploidy) occurs frequently in plants– autopolyploidy (without hybridization)– allopolyploidy (with hybridization)

Autopolyploid Speciation

Increase in ploidy due to unreducedgametes and selfing.

Autopolyploidy

Species ASpecies A(2n = 8; 2X)

Species A(2n = 8; 2X)

Species A?(2n = 16; 4X)

Meiotic error &selfing occur.

Normal meiosis &selfing orout-crossingoccurs.

Autopolyploidy

• Autopolyploidy results in a form of reproductive isolation, but often there is no accompanying morphological or ecological differentiation

• Many species with autopolyploidy are considered to include autopolyploid races; in other cases, distinct species are recognized based on ploidy levels if morphological/ecological differentiation has occurred

Allopolyploid Speciation

AD

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Allopolyploidy

Species A(AA)

X

Species D(DD)

Hybrid AD(infertile &cannot crosswith eitherparent)

gamete A

gamete D

Allopolyploidy

Hybrid AD

Chromosomedoubling AADD (now sex

cells-AA, AD, DD-can be produced!)

Parent AA

X

AADD

AAD (infertilebut could persistthrough vegetativereproduction)

gamete A

gamete AD

Allopolyploidy in Tragopogon

1) DD 2) PP

3) RR

PR

PD

DR

4) DDRR

5) PPDD

PR

2) T. pratensis

1) T. dubius

3) T. porrifolius

4) T. mirus

5) T. miscellus

See Box 6Ein the textfor moreinformation.

Many plant hybrids are possible

American and Oriental plane trees (Platanus occidentalis and P. orientalis, respectively) have been geographically isolated for at least 20 MY, but when artificially hybridized are still able to form a fully fertile hybrid, the London plane, which is used as a smog-resistant planter species in major North American cities..

Naturally-occurring plant hybrids: many examples

Penstemon (Scrophulariaceae)

The World Famous...

“Brassica Triangle”

An example of hybrid, polyploid speciation in wheat, Triticum aestivum (Poaceae)

Polyploid Crops

• Wheat – 6X

• Potatoes – 4X

• Maize – 4X

• Sugarcane – 10X or 12X

• Sweet Potato – 6X

• Cotton – 4X

• Banana – 3X

Polyploidy in Tracheophyte Evolution

• Recent studies have shown that it is likely that at least 95% of all ferns have evidence of polyploidy in their lineages.

• Estimated that as much as 75% of all angiosperms have at least one episode of polyploidy at some point of their evolutionary history!

Modes of Speciation

sympatric speciation = reproductive isolation ofparent species fromtheir derivatives throughhybridization and chromosome doubling (or justchromosome doubling)without geographic isolation

Modes of Speciation: Summary

• Allopatric speciation

• Local speciation

• Sympatric speciation– Autopolyploidy (without hybridization)

– Allopolyploidy (with hybridization)

Geographic or habitatisolation accompanies reproductive isolation

Reproductive isolationoccurs without geographic isolation

Q: “What is a species?”

Extensive variation in species concepts among biologists:

• Biological – Gap in interfertility – “reproductive isolation”• Recognition – Common fertilization system• Phenetic – Gap in variation between species (discontinuity)• Evolutionary – Common evolutionary fate through time• Cladistic – Defined by an apomorphy as being monophyletic• Taxonomic/Diagnostic – Unique combination of characters• Genealogical – Being more closely related to each other than to members of any other group

Biological Species Concept

• Relies upon the concept of post-divergence reproductive isolation (lack of gene flow through allopatric speciation)

• Assumes that species cannot interbreed

• In general, works well for vertebrates

• Well………This doesn’t work for plants!!

Morphological Species Concept

• the idea that one can observe either special, uniquely derived characters or identify morphological discontinuities between putatively related organisms

• Need to take into account the range of morphological variation

• Since diversity is present in all populations, it is implicit to somehow describe the diversity, and then evaluate any discontinuities

• Ecological differentiation is usually also included as a criterion

• A ‘practical’ concept, especially where identification is the primary goal

Which is the best species concept to use for plants?

• How do you evaluate what is “best” for your situation?

• Do you have enough information to use all of the species concepts, or do some require more information than you have available?

• In practice, some form of the morphological species concept is generally used but may be complemented by molecular/genetic data when available

But it is important to know your organisms—what biological processes might have an impact on variation?

Morphological intermediacy may well be an indicator ofhybridization, but could be due to clinal variation orother processes.

Figure 6.18 from the textFigure 6.18 from the text